"Low profile" forage harvesters employing only a single level of gathering conveyors to transport the severed crop stalks upwardly and rearwardly into the chopper are typically provided with overhead crop guidance structure of one form or another to assist in controlling the tops of the stalks while the lower ends are controlled by the conveyors. Although the conveyors converge the lower ends of the stalks laterally so as to facilitate delivery through a common outlet into the chopper, some overhead assistance is required by guidance structure to force the tops of the stalks to likewise converge laterally for passage through the outlet. Typically also, conventional harvesters are provided with so-called "knock-down bars" or "lean bars" which engage the standing crop stalks as they move toward the outlet and lean the stalks back toward the front of the harvesting header so that the butts of the stalks become presented to the outlet. Hence, delivery of the stalks through the outlet is in a butt-first condition so that the stalks can then be chopped accurately into countless, rather short segments by the chopping mechanism.
While in principle currently available overhead guidance apparatus of the aforementioned type is adequate, it is largely designed for use in uniform, unchanging crop conditions, e.g., crop stalks of uniformly one height, diameter, leafiness, fruit yield, etc. In reality, however, the harvesting operator frequently encounters variations from field-to-field and regions of varying crop conditions in each individual field being harvested. Thus, constructing the guidance apparatus according to preestablished specifications based upon an "average" yield and, an "average" stalk height dooms the apparatus to inefficient operation and, conceivably, to complete operational shutdown, if the crops actually being harvested vary substantially from the preestablished norm.
Accordingly, preadjustment of the guidance apparatus based upon the harvesting operator's analysis of conditions in a particular field is very desirable. But that alone is insufficient because the guidance apparatus still needs the ability to "self-adjust" to a certain degree when encountering substantial changes in crop conditions within the same field. To date, this ability has simply not existed.